Although several studies have characterized the plasma levels of progesterone and estradiol-17.beta. during pregnancy and lactation in the dog, there have been no studies of the polypeptide hormone, relaxin, in this species. Relaxin is a homologue of insulin that plays an important role in pregnancy maintenance and remodeling the reproductive tract in preparation for parturition in many mammalian species (please see reviews by Schwabe et al., 1978, Recent Prog. Horm. Res., 34:123-199; Bryant-Greenwood, 1982, Endocr. Rev., 3:62-90; MacLennan, 1983, Clin. Reprod. Fert. 1., 2:77-95; Weiss, 1984, Annu. Rev. Physiol., 46:43-52; Kemp and Niall, 1984, Vitam. Horm., 41:79-115). A product of the corpus luteum in certain species and endometrium or placenta in others, relaxin is known to inhibit uterine myometrial contractions, augment uterine growth to accommodate the growing fetuses and to increase the flexibility of the pelvic girdle and the distensibility of the uterine cervix in preparation for parturition (Schwabe et al., 1978, supra; Downing and Sherwood, 1985a, Endocrinology, 116:1200-1205; 1985b, Endocrinology, 116:1206-1214; 1985c, Endocrinology, 116:1215-1220). In harmony with the projected role of relaxin in mammalian pregnancy, plasma levels of this hormone increase in the latter half of pregnancy in many species (horse, cat, pig, rat, mouse, guinea pig, hamster: Sherwood et al., 1975, Endocrinology, 97:834-837; O'Byrne and Steinetz, 1976, Proc. Soc. Exp. Biol. Med., 152:272-276; O'Byrne et al., 1976, Endocrinology, 99:1333-1335; Stewart and Stabenfeldt, 1981, Biol. Reprod., 25:281-289; 1985, Biol. Reprod., 32:848-854) and then decline just before parturition. In women and nonhuman primates, plasma relaxin is detected throughout pregnancy with no obvious peak (O'Byrne et al., 1978, J. Clin. Endocrinol. Metab., 47:1106-1110; Weiss et al., 1981, Biol. Reprod., 24:565-567; Castracane et al., 1983, In: Grew et al (eds.) Factors Regulating Ovarian Function, New York: Raven Press pp 415-419). Although relaxin has been shown to enhance growth of the mammary glands (Schwabe et al., 1978, supra, for older literature; Bani et al., 1985, J. Endocrinol. Invest., 8:207-215; 1986, J. Endocrinol. Invest., 9:153-158; Bianchi et al., 1986, J. Endocrinol. Invest., 9:153-158), a role in lactation has not been proven; indeed, with the exception of one laboratory (Bryant and Chamley, 1976, J. Reprod. Fertil., 46:457-459; Afele et al., 1979, J. Reprod. Fertil., 56:451-457; Whitely et al., 1985, Biol. Reprod., 33:705-714), most investigators have not detected relaxin in the peripheral plasma for more than 24-48 h after parturition or during the lactation period in any of the above species (Sherwood et al., 1975, Endocrinology, 97:834-837 & 96:1106-1113; 1980, Endocrinology, 107:691-698; 1981, Biol. Reprod., 255:65-71; O'Byrne and Steinetz, 1976, supra; O'Byrne et al., 1976, supra; 1978, supra; Stewart and Stabenfeldt, 1981, supra; 1985, supra; Weiss et al., 1981, supra; Castracane et al., 1983, supra).
Relaxin has been shown to be present in pregnant cats using antibodies described herein (see Addiego et al, Biol. of Reprod., 37:1165 Dec., 1987). The presence of pseudopregnancy in cats is described by Olson et al, Vet Clin. N. Am. Small Anim. Prac., 14:927 (1984)).